In the description of the Kraken race, it says they gain the benefit of 10 power points if they start as elementalists, or "if they later become elementalists".

Does this mean that the Background feat Arcane Background can be taken after character creation if you are a Kraken?

Does it also mena that the Background feat Arcane Background can be taken after character creation if you are a human, masquatani or scurrilan?

Pretty much any Background Edge can be taken after character creation. It's covered under the second paragraph under Background Edges on page 22 of SWR. One example is specifically of picking up Arcane Background. The player just needs a good reason and GM approval.